Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images which show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions which emanate from the body and are captured by a scintillation crystal, with which the photons interact to produce flashes of light or “events.” Events are detected by an array of photodetectors, such as photomultiplier tubes, and their spatial locations or positions are calculated and stored. In this way an image of the organ or tissue under study is created from detection of the distribution of the radioisotopes in the body.
In nuclear imaging, a patient is injected with or swallows a radioactive isotope which has an affinity for a particular organ structure or tissue of the body. Gamma rays are then emitted from the body part of interest are collimated by a collimator so that only gamma photons travelino in a direction perpendicular to the surface of a detector head are allowed to impinge on the detector head, and are detected by a gamma camera apparatus including the detector head, which forms an image of the organ based on the detected concentration and distribution of the radioactive isotope within the body part of interest. Nuclear images may be obtained using SPECT (Single Photon Emission Computed Tomography), or PET (Positron Emission Tomography). SPECT produces multiple image “slices” from single gamma photons, each slice representing a different plane in a three-dimensional region such that when the slices are considered collectively, a three-dimensional image of the region may be studied.
PET is used to produce images for diagnosing the biochemistry or physiology of a specific organ, tumor or other metabolically active site. Measurement of the tissue concentration of a positron emitting radionuclide is based on coincidence detection of the two gamma photons arising from positron annihilation. When a positron is annihilated by an electron, two 511 keV gamma photons are simultaneously produced and travel in approximately opposite directions. Gamma photons produced by an annihilation event can be detected by a pair of oppositely disposed radiation detectors capable of producing a signal in response to the interaction of the gamma photons with a scintillation crystal. Annihilation events are typically identified by a time coincidence between the detection of the two 511 keV gamma photons in the two oppositely disposed detectors, i.e., the gamma photon emissions are detected virtually simultaneously by each detector. When two oppositely disposed gamma photons each strike an oppositely disposed detector to produce a time coincidence event, they also identify a line of response, or LOR, along which the annihilation event has occurred. An example of a PET method and apparatus is described in U.S. Pat. No. 6,858,847, which patent is incorporated herein by reference in its entirety.
Another known tomography system is computed axial tomography (CAT, or now also referred to as CT, XCT, or x-ray CT). In CT, an external x-ray source is caused to be passed around a patient. Detectors around the patient then respond to the x-ray transmission through the patient to produce an image of the area of study. Unlike PET, which is an emission tomography technique because it relies on detecting radiation emitted from inside the patient, CT is a transmission tomography technique which utilizes a radiation source external to the patient. CT provides images of the internal structures of the body, such as the bones, whereas PET, as described above, provides images of the functional or metabolic aspects of organs or tissues of the body.
A CT scanner uses a similar mechanical setup as a PET scanner. However, unlike PET, a CT scanner requires a source of x-ray radiation mounted opposite a detector. In third-generation computed tomography systems, the CT detector and x-ray source are mounted on diametrically opposite sides of a gantry which is rotated around the patient as the patient traverses the tunnel of the gantry.
The x-ray source of a CT imaging device typically emits a fan-shaped beam of x-rays which pass through the patient and are received by an array of detectors. As the x-rays pass through the patient, they are attenuated as a function of the densities of objects in their path. The output signal generated by each detector is representative of the electron densities of all objects between the x-ray source and the detector.
The CT detectors can utilize scintillator crystals which are sensitive to the energy level of the x-rays. Multiple light pulses produced by each scintillator crystal as it interacts with the x-rays are integrated to produce an output signal which is related to the number of the x-rays sensed by the scintillator crystal. The individual output signals are then collectively processed to generate a CT image. Other detectors can be used in CT tomographs. For example, a solid state silicon diode can be used to detect the low energy x-rays directly.
CT imaging is generally suited for providing anatomical and structural information, whereas PET is more adept for studying the biochemistry or physiology of a specific organ, tumor or other metabolically active site. Consequently, it is particularly useful in certain studies such as oncological, neurological and cardiovascular studies to use PET imaging for diagnostic purposes, and to align or register the nuclear image with a medical image from another modality such as CT, which offers better anatomical information. Such a fused image, for example, enables clinicians to determine the anatomical position of a lesion displayed by the nuclear image more accurately and the organs and structures that are affected can be ascertained with a higher degree of accuracy and confidence.
Hybrid imaging devices, which combine the functional sensitivity of PET with the anatomical detail of diagnostic multi-slice CT in a single, integral imaging device generally are known in the art, see, e.g. U.S. Pat. Nos. 6,449,331, 6,490,476 and 6,631,284, incorporated herein by reference in their entirety. However, such integrated devices are costly and impractical for diagnostic service providers that already possess stand alone-type CT imaging devices. Indeed, it may not be economically feasible for a diagnostic service provider to purchase a new, integrated hybrid device when such individual already possesses a stand alone-type CT imaging device. Additionally, in many instances, clinicians may have already constructed special buildings or rooms with which to house their existing stand alone-type CT imaging device such that the purchase of a new hybrid device may require the demolition and/or construction of a new building or room—which can be undesirable and/or cost prohibitive. Consequently, there is a need for a mobile compact SPECT imaging device that can be retrofitted with an existing stand alone-type CT imaging device to thereby form a hybrid device.